The Heil Tube is the forerunner
of the Klystron. In 1935 A. Arsenjewa-Heil and O. Heil, a husband and
wife team, made the tube as an RF oscillator. It works by velocity modulation
of an electron beam. An RF voltage applied across the interaction region
between the focus electrode and the drift tube causes this velocity modulation.
Faster electrons from one cycle catch up with slower electrons from the
previous cycle, which causes bunching of the electrons in the beam by
the time they reach the interaction region between the drift tube and
the second electrode.